The present invention relates to a method and apparatus for building an equalized tobacco stream wherein the weight of each unit length or increment closely approximates or matches a predetermined weight. More particularly, the invention relates to improvements in a method and apparatus for building a continuous equalized tobacco stream by weighing successive increments of an unequalized stream and by changing the speed of movement of such increments along a predetermined portion of an elongated path when the weight of certain increments or series of increments deviates from the predetermined weight. Still more particularly, the invention relates to improvements in tobacco feeding and weighing devices of the type known as BZO and DWB (manufactured and sold by the assignee of the present application).
The weighing device of the type known as DWB comprises an endless conveyor which transports a series of unit lengths of a tobacco stream along a portion of the path for tobacco leaves or the like. The conveyor of the weighing device receives successive unit lengths from a conveyor of the feeding device. The weighing device further includes means for monitoring the weight of successive unit lengths on its conveyor and means for varying the speed of such conveyor when the monitored weight deviates from the predetermined weight. This can be achieved by multiplying first signals denoting the weight of successive unit lengths with second signals denoting the speed of corresponding unit lengths on the conveyor of the weighing device, and varying the speed of such conveyor so that the intensity or another characteristic of third signals denoting the products remains constant. The speed of the conveyor in the feeding device is a function of the speed of the conveyor which forms part of the weighing device. The conveyor which receives the equalized stream from the conveyor of the weighing device is preferably driven at a constant speed.
All operations which are to be performed by the components of a modern production line for the treatment of tobacco leaves prior to destalking are normally carried out in a fully automatic way. As a rule, such components should receive and process a continuous tobacco stream wherein the weight of each unit length matches or closely approximates a predetermined weight. This simplifies the controls of the production line because each and every automatically controlled unit invariably receives identical quantities of tobacco per unit of time. Therefore, such production lines embody weighing devices which are designed to furnish signals denoting the weight of successive unit lengths of the stream, and means for changing the weight when the monitored weight deviates from the desired (predetermined weight). It has been found that equalization of the stream contributes significantly to predictable and optimum treatment (such as conditioning (changing the moisture content) and destalking) of tobacco leaves or the like.
The manner in which the speed of the conveyor of the feeding device is regulated as a function of the speed of the conveyor in the weighing device is such that the throughput of the weighing device (e.g., in kilograms per hour) is constant. A combination of a feeding device and a weighing device which is often used in presently known production lines is shown in FIG. 4 of U.S. Pat. No. 3,903,901. The patented combination can compensate for deviations of the throughput (e.g., in kg/hr) from the average value by .+-.75 percent, i.e., the speed of conveyors in the two devices can be changed by .+-.75 percent. It has been found that such range of adjustments is often too narrow, i.e., that it does not suffice to compensate for differences in specific weight of various types of tobacco and/or for differences in the size and/or composition (consistency) of portions of such tobacco. In many instances, the differences between specific weights of different types of tobacco are so pronounced that, even if the tobacco is supplied in such a way that the volumes of successive increments which are delivered to the conveyor of the weighing device are identical, the one or the other limit of the aforementioned range is reached (and normally exceeded) whenever the production line receives a different type of tobacco.
Another reason for insufficiency of the aforediscussed range of adjustments (.+-.75 percent) is that the volume of a stream which consists of a first type of tobacco and is delivered by the feeding device subsequent to delivery of tobacco of a different second type is quite different from the volume of the previously delivered stream. Additional reasons for insufficiency of the aforediscussed range are differences in consistency and/or size of particles of different types of tobacco. As explained above, the inability of the combination of feeding and weighing devices to invariably supply identical quantities of tobacco per unit of time can greatly affect the quality of treatment of tobacco in a production line which extends to or includes the destalking unit.
Attempts to compensate for the aforediscussed relatively narrow range of adjustments include the provision of devices which allow for changes in the transmission ratio of drive means for the conveyors of the feeding and weighing devices. Such transmissions often include chains for transmission of torque between the two conveyors. The chains are replaced with different chains whenever the production line is to receive a different type of tobacco. This is a time-consuming operation which invariably entails complete stoppage of all mobile parts of the production line. Such stoppage causes pronounced losses in output, especially if the processed material is to be supplied directly to a production line for the mass-manufacture of cigarettes or other rod-shaped articles constituting or forming part of smokers' products. Moreover, the upper or lower limit of the aforediscussed range can be reached even if the nature of supplied tobacco is not changed at all. This can occur as a result of pronounced differences between the consistency, particle size, moisture content and/or another characteristic of successive batches of one and the same tobacco type.